By suitable mass distribution as between the two coaxial parts of the torsion damper, the resonant frequency of the latter (which is typically of the kind commonly referred to as a double damped flywheel) can be arranged, with advantage, to have a value below the frequency of vibration occurring in the slow running mode of the engines of most motor vehicles. This enhances the comfort of the occupants of the vehicle at all engine speeds. However, on starting and stopping, it is important to resist oscillations, which may have a high amplitude, which may occur between the two coaxial parts at the resonant frequency. It is for this reason that the above mentioned friction means are provided between the two parts of the damper.
In the specification of German published patent application DE 3 931 429A, the said friction means comprise a friction ring which is only effective beyond a particular amount of angular displacement between the two coaxial parts of the damper. Thus there is no frictional damping of low amplitude oscillations of the coaxial parts, though on the other hand the desired frictional damping does occur when oscillations of higher amplitude take place.
However, in the above mentioned published German patent document, this friction ring works in the axial direction, so that the resilient means which are commonly provided for the purpose of loading the friction ring must of necessity also work in the axial direction. Having regard to the amount of space available, the deflection of these resilient means is of necessity very restricted.
Control of the load on the friction ring is accordingly somewhat poor, as a result of which it is almost impossible to mitigate the consequences of its inevitable wear.